1. Field of the Invention
The present invention relates to a method of driving an active matrix liquid crystal display, particularly relates to a method of driving a liquid crystal display according to a pulse width modulation driving system in which a thin-film transistor is used for a switching device and a liquid crystal display driven by the driving method.
2. Description of the Prior Art
An active matrix liquid crystal display that executes gradation display according to a pulse width modulation (PWM) system is known. In a method of driving the liquid crystal display according to the PWM system, display according to each gradation is realized by not varying a level of gradation voltage applied to a display pixel according to displayed gradation but varying duration in which voltage is applied according to displayed gradation. In the liquid crystal display according to the PWM driving system, the number of display gradations can be increased, using a gradation voltage generating circuit having simple configuration.
Such a method of driving the liquid crystal display according to the PWM driving system proposed in Japanese published unexamined patent application No. Hei4-142592 will be described as an example below. FIG. 6 is a block diagram showing a main part of a conventional type liquid crystal display.
The conventional type liquid crystal display is provided with plural scanning lines 101 and plural signal conductors 102 respectively crossing each other, plural pixel electrodes 103 arrayed in parts in which these are crossed, a liquid crystal display panel 100 where each thin-film transistor 104 which is connected to each pixel electrode and which executes switching operation when a gate signal is sent to the corresponding scanning line 101 and a data signal is sent to the corresponding signal conductor 102 is formed, a scanning line driving circuit 105 that applies a gate signal to the plural scanning lines 101 and a signal conductor driving circuit 106 that sends a data signal applied to each pixel to the plural signal conductors 102. The scanning line driving circuit 105 sequentially selects plural scanning lines 101 in one vertical term, turns on plural thin-film transistors, and the signal conductor driving circuit 106 applies a data signal to each of plural pixel electrodes 103 connected to each of the turned-on plural transistors only in a term of pulse width determined according to displayed gradation.
FIG. 7 is a timing chart showing the variation of applied voltage VP applied to a certain pixel electrode 103 of the liquid crystal display according to the PWM driving system shown in FIG. 6. When a gate signal VG applied to the scanning line 101 is turned at a high level VGH while a data signal VD applied to the signal conductor 102 in a certain one vertical term (1V) is at a reference level, the thin-film transistor 104 is turned on, applied voltage VP is increased in accordance with the data signal VD and further, when the data signal VD is turned at a positive active level, the applied voltage VP is increased in accordance with the data signal only in a term TW1 of pulse width determined according to displayed gradation. Next, when the gate signal VG is turned at a low level VGL, the thin-filmtransistor 104 is turned off and applied voltage VP at this time is held. When the gate signal VG applied to the scanning line 101 is turned at a high level VGH while the data signal VD applied to the signal conductor 102 in the next one vertical term is at the reference level, the thin-filmtransistor 104 is turned on, applied voltage VP is decreased in accordance with the data signal VD and further, when the data signal VD is turned at a negative active level, applied voltage VP is decreased in accordance with the data signal VD only in a term TW2 of pulse width determined according to the displayed gradation. Next, when the gate signal VG is turned at a low level VGL, the thin-firm transistor 104 is turned off and applied voltage VP at this time is held. As described above, multi-gradation display is realized, using the gradation voltage generating circuit having simple configuration by turning the data signal at the active level only in pulse duration according to displayed gradation.
In the Japanese published unexamined patent application No. Hei4-142592, to prevent voltage applied to a pixel electrode and having data polarity for writing from being asymmetrical and to prevent flickering and burning from being caused, it is also proposed that if gate-on voltage in case a data signal is negative is VGONN and gate-on voltage in case the data signal is positive is VGONP, VGONN is set so that it is lower than VGONP.
By the way, the temperature and the display characteristic of the liquid crystal display panel of such a liquid crystal display according to the PWM driving system will be reviewed below. The ON state-current of each thin-film transistor which is connected to each pixel electrode and which executes switching operation depends upon the temperature of the panel and as the temperature of the panel rises, the ON-state current increases. As voltage applied to a liquid crystal is proportional to the product of the ON-state current and data signal pulse width, the displayed gradation-luminance characteristic of the liquid crystal display panel varies according to the temperature of the panel. Therefore, when the temperature of the panel varies, the display image quality of the liquid crystal display panel varies. Besides, as the electric characteristics of the thin-film transistor depend upon the temperature of the panel, the asymmetric property of writing quantity according to data polarity is different depending upon the temperature of the panel and when the temperature of the panel varies, flickering and burning are caused.